The Suomi National Polar-orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) NASA standard Level-2 (L2) water vapor products provide clear-sky total column water vapor (TPW) properties to continue to depict global moisture at a high spatial resolution started with MODIS-derived data on the Terra and Aqua platforms. While MODIS has two water vapor channels within the 6.5 μm H2O absorption band and four channels within the 15 μm CO2 absorption band, VIIRS has no channels in either infrared (IR) absorption band. However the VIIRS IR window channels at 8.6, 10.8 and 12 µm provide some indication of low-level moisture; to compensate for the absence of water vapor absorption channels, this product leverages water vapor soundings from two additional sounder instruments aboard the SNPP platform: Advanced Technology Microwave Sounder (ATMS) and Cross-track Infrared Sounder (CrIS). The Water Vapor (WATVP) algorithm was adapted from the MODIS MOD07 atmospheric profile algorithm, with adjustments to accommodate for the absence of some sounding spectral bands by adding the CrIS+ATMS TPW soundings, and to realize the advantage of greatly increased spatial resolution (750 m) with good radiometric signal to noise (better than 0.35 K for typical scene temperatures in all spectral bands).
The nominal spatial resolution of the SNPP VIIRS L2 Cloud mask is 750 meters.
The L2 water vapor product, in Network Common Data Form (netCDF), is acquired every 6 minutes, and contains the following 8 Science Data Sets (SDS):
- Atmosphere water vapor content (collocated NUCAPS1 retrieval)
- Atmosphere water vapor content (VIIRS + NUCAPS retrieval)
- Atmosphere water vapor content (VIIRS-only retrieval)
- Clear-sky confidence
- Model atmosphere water vapor content
- Model surface pressure
- Quality flag
- Surface skin temperature
The temporal range for this SNPP VIIRS L2 product starts from May 1, 2012 (Day-of-year: 122). Currently, this collection is available through September 11, 2018 (Day-of-year: 254); this cutoff date is driven by the availability of NUCAPS, which itself is based on CrIS and ATMS inputs. Version-1 defines the current collection for the water vapor suite that is available from the LAADS DAAC. Consult the SNPP VIIR Water Vapor product user guide and Algorithm Theoretical Basis Document for additional information regarding the water vapor product suite, its algorithm, global attributes, data field attributes, SDS names and descriptions, quality flags, etc.
- NUCAPS: NOAA Unique CrIS ATMS processing System: A NOAA-developed system that produces: (1) spectrally and spatially thinned radiances, (2) retrieved products such as profiles of temperature, moisture, trace gases and cloud-cleared radiances, and (3) global validation products such as radiosonde matchups and gridded radiances and profiles.